This application relates to turbine blades and in particular relates to improved turbine blade tip clearance characteristics.
Turbine engines include a compressor for compressing air that is mixed with fuel and ignited in a combustor for generating combustion gases. The combustion gases flow to a turbine such that thermal energy produced within the combustor is converted into mechanical energy within the turbine by impinging the hot combustion gases onto one, or alternatively, a series of bladed rotor assemblies.
The performance and efficiency of turbine engines are critically affected by the clearances that exist between the rotating and stationary components within the turbine. As the clearances increase between the bladed rotor assemblies and the stationary assemblies, such as shrouds, the efficiency of the turbine decreases.
Accordingly, it is desirable for a turbine designer to maintain the clearances, herein referred to as "clearance gaps", between the bladed rotor assemblies and the shroud at a minimum without interfering with the rotation of the rotor assembly or affecting the structural integrity of the rotor or shroud. Even with sophisticated clearance control methods, however, clearance gaps cannot be completely eliminated.
The clearance gaps between the tip of the rotor blades and the adjacent stationary shrouds provide a narrow flow passage between the pressure and suction sides of a blade, resulting in hot gas flow leakage that is detrimental to the blade aerodynamic performance. Although the resulting leakage flow is undesirable, the clearance gaps must accommodate for the overall growth of the blade during operation. The overall growth of the blade is a product of several growth components including thermal expansion of the rotor, which expansion results because the rotor is typically more difficult to cool than the shroud. This cooling difficulty arises because the rotor blade extends over a relatively large radial distance and involves the thermal expansion of many sections, whereas the shroud is a much more compact component.
As beforementioned, the primary detrimental effect of the tip leakage flow is on the blade aerodynamic performance but a second important and less well understood effect concerns the convection heat transfer associated with the leakage flow. Surface area at the blade tip in contact with the hot working gas represents an additional thermal loading on the blade which, together with heat transfer to the suction and pressure side surface area, must be removed by the blade internal cooling flows. The additional thermal loading imposes a thermodynamic penalty on engine performance and degrades overall turbine performance.
The resultant thermal loading at the blade tip can be very significant and detrimental to the tip durability, especially the blade tip region near the trailing edge, which region can be difficult to cool adequately with blade internal cooling flows. As a result, blade tips have traditionally been one of the turbine areas most susceptible to structural damage. Structural damage to the blade tips can have a severe effect on turbine performance. Loss of material from the tip increases the clearance gap, increases the leakage flow and heat transfer across the tip, and in general exacerbates all of the above problems.
Numerous conventional blade tip designs exist for maintaining the proper pressure and suction side flow surfaces of the blade at the tip cap as well as providing minimum clearances with the stator shroud. Numerous cooling configurations also exist for cooling the blade tip caps for obtaining useful lives of the blades without undesirable erosion. Since cooling of the blade, including the blade tip, uses a portion of the compressed air from the gas turbine compressor, that air is unavailable for combustion in the combustor of the engine which decreases the overall efficiency of the turbine engine. Accordingly, the cooling of the blade including the blade tip should be accomplished with as little compressed air as possible to minimize the loss in turbine efficiency.
Therefore, it is apparent from the above that there exists a need in the art for improvements in turbine blade tip leakage flow characteristics.